Field of the Invention
The present invention describes a process for preparing an N-methyl-substituted triacetonamine compound (triacetonamine=“TAA”). The present invention particularly describes a process for methylating the nitrogen present in the ring, as shown, for example, in the reaction equation <1> (where R is, for example, an alkyl radical, an alkoxy radical, an amine radical or else an OH group):

Discussion of the Background
Methylated derivatives of 2,2,6,6-tetramethylpiperidine are employed with particular significance as “hindered amine light stabilizers” in particular. The methylation enables employment under acidic conditions as well. Commercial products having N-methylated 2,2,6,6-tetramethylpiperidine groups are, for example, Tinuvin® 292 [a mixture of 1-(methyl)-8-(1,2,2,6,6-pentamethyl-4-piperidinyl)sebacate and bis(1,2,2,6,6-pentamethyl-4-piperidinyl)sebacate] or Cyasorb® UV-3529 (CAS NUMBER 193098-40-7).
The related art discloses various methods of methylating amines.
For example, Kopka, I. E., et al., J. Org. Chem. 1980, 45, 4616-4622 and Minatti, A., et al., J. Org. Chem. 2007, 72, 9253-9258 describe the reaction of amines with methyl halides. This is shown in schematic form in reaction equation <2> (where R is a radical as defined in relation to reaction equation <1>):

A disadvantage of the method shown in reaction equation <2> is that it is necessary to use at least one equivalent of a suitable base in addition to the methyl halide to release the product. This additionally leads to formation of the corresponding salts, which then arise as a waste product. An additional problem is that selective alkylation to give the tertiary amine is generally impossible since overalkylation to give the corresponding quaternary ammonium salt can take place.
A further method described in the related art is the Eschweiler-Clarke reaction (e.g. Lutz, W. B., et al., J. Org. Chem. 1962, 27, 1695-1703; EP 0 729 947 A1; WO 2004/072035 A1; WO 2005/123679 A2). In this method, the amine is reacted with formaldehyde in the presence of formic acid. The formic acid functions as a reducing agent and is converted to CO2. In order that the reaction proceeds, one equivalent of base is generally additionally required. The reaction is illustrated schematically in the following reaction equation <3> (where R is a radical as defined in relation to reaction equation <1>):

A disadvantage here is the need to use formic acid. Moreover, the use of the base again leads to generation of a corresponding waste stream.
A further means of N-methylation described in the related art (e.g. WO 2004/089913 A1, WO 2008/101979 A1) is the reaction with formaldehyde in the presence of borohydrides (e.g. sodium borohydride). The reaction is illustrated schematically in the following reaction equation <4> (where R is a radical as defined in relation to reaction equation <1>):

A disadvantage here is the need to use the borohydrides. The workup gives rise to large amounts of boric acid or boric acid derivatives as a waste stream.